Bandpass filter for passing a wide range of frequencies and suppressing a narrow range of frequencies

ABSTRACT

1,022,306. Electrical filter networks; multiplex radio signalling. PHILIPS ELECTRONIC &amp; ASSOCIATED INDUSTRIES Ltd. Dec. 30, 1963 [Jan. 2, 1963], No. 51137/63. Headings H3U and H4L. A band-pass filter for passing a comparatively wide frequency range and also for suppressing a comparatively narrow frequency band adjacent to said wide range, comprises series branches I, II, each constituted by a seriesresonant circuit tuned to the mid-point of the wide range and a shunt branch comprising individual branches III, IV each comprising series-resonant circuits tuned to the narrow band, the branch IV being coupled across capacitor 35 forming part of branch III. The input signal which may bea television intermediate-frequency signal is applied to input terminals on the left and the output signal from which signals, e.g. sound carrier, within the comparatively narrow band have been suppressed appears across capacitor 21 which may be the input capacitance of a vision intermediate frequency amplifier valve, Fig. 3 (not shown). The corresponding sound carrier signal, i.e. the comparatively narrow frequency band, is developed across capacitor 41 which may be the input capacitance of the sound I.F. amplifier. To obtain the band-pass characteristics inductors 13, 23 and 33, 43 may be inductively coupled though this is not essential and in the preferred embodiment inductive coupling is not used. Resistor 40 serves as a neutralizing component to reduce the sharpness of the narrow frequency band.

DeQ 1967 A. BENSASSON 3,358,246

BANDPASS FILTER FOR PASSING A WIDE RANGE OF FREQUENCIES AND SUPPRESSING A NARROW RANGE OF FREQUENCIES Filed Dec. 13, 1963 PRIOR ART VIDEO OUTPUT SOUND OUTPUT FIG.4

PRIOR ART INVENTOR.

ALBERT BENSASSON w/e. l8

.. AGENT United States Patent 3,358,246 BANDPASS FILTER FOR PASSING A WIDE RANGE OF FREQUENCIES AND SUPPRESSING A NAR- ROW RANGE OF FREQUENCIES Albert Bensasson, Bougival, France, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Dec. 13, 1963, Ser. No. 330,362 Claims priority, application France, Jan. 2, 1963, 920,307 7 Claims. c1. 333-6) ABSTRACT OF THE DISCLOSURE A bandpass filter is provided having a pair of series connected tuned longitudinal branches, and a transverse branch connected to the junction of the series branches. The longitudinal branches are tuned to have a low impedance to a wide band of frequencies. The transverse branch is a series resonant circuit tuned to a narrow band in or near the wide band, so the filter rejects the narrow band. A fourth branch comprised of a series resonant circuit tuned to the narrow band is connected in parallel with part of the transverse branch in order to increase the width of the narrow band.

This invention relates to bandpass filters for passing a comparatively wide range of frequencies and also for strongly and selectively suppressing a comparatively narrawo range of frequencies located within the said range or in the vicinity thereof, comprising two longitudinal branches connected in series and each comprising a seriesresonant circuit and a transverse branch coupled to the common point of the longitudinal branches and comprising a series-resonant circuit which is tuned to a frequency located in the comparatively narrow range of frequencies.

Such filters are used inter alia in television receivers as intermediate-frequency bandpass filters. The comparatively wide range of frequencies comprises the intermediatefrequency video-signal and the comparatively narrow range of frequencies comprises the sound carrier wave. As is well-known, the sound carrier wave must in many cases be considerably suppressed relative to the intermediate-frequency video-signal in the intermediate-frequency portion of the receiver. Also the sound carrier wave is in many cases derived from said intermediatefrequency portion.

The invention has for its object to provide an improvement in such filters. To this end, the band-pass filter according to the invention is characterized in that parallel with a portion of the transverse branch there is connected a further branch which likewise comprises a series-resonant circuit which is tuned to the frequency located in the comparatively narrow range of frequencies.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which:

FIGURE 1 shows a bandpass filter of known type;

FIGURE 2 shows a bandpass filter according to the invention;

FIGURE 3 shows a circuit including a bandpass filter according to the invention;

FIGURE 4 shows a response curve of the filter of FIG- URE 1, and

FIGURE 5 shows a response curve of a bandpass filter according to the invention.

The known filter shown in FIGURE 1 is built up from:

a primary branch I, comprising the series-combination of a capacitor 11, a resistor 12 and an inductor 13;

3,358,246 Patented Dec. 12, 1967 a second branch II, comprising the series-combination of a capacitor 21, a resistor 22 and an inductor 23;

a third branch III, comprising the series combination of a capacitor 31, a resistor 32 and an inductor 33.

The branch III is tuned to the frequency to be eliminated whereas the branches I and II behave as a transformer of which I is the primary winding and II is the secondary winding. In practice the two branches I and II are usually tuned to the same frequency, the width of the pass band, as is well-known, being substantially de termined by the value of the coupling coefficient. Such a network shows a response curve as illustrated in FIG- URE 4, in which the response A is plotted as a function (f) of the frequency f. The figure shows that said curve has two relative maxima M1 and M2, which are as far remote from each other as possible, and between them a relative minimum m. In practice, it is ensured that the two relative maxirna of the function (f) have the same ordinates and that the relative minimum difiers from this function as little as possible.

It is known that with such a filter the response for the tuning frequency f of the branch III may be reduced at will by means of a neutralizing resistor 30. However, in spite of these advantages, such a filter suifers from a severe drawback: the suppression range of the response curve is comparatively narrow and this fact may hazard the efficaciousness of the suppression, eg if frequency drift of the local oscillator occurs. The narrowness of the suppression range can be seen in FIGURE 4 from the fairly acute character of the angle or shown therein.

The aforementioned disadvantage is avoided in the filter according to the invention as shown diagrammatically in FIGURE 2.

This filter includes, relative to the filter shown in FIG- URE 1, a fourth branch IV which is tuned to the same frequency f as the branch III of the filter previously described and which is capacitively coupled to the branch III. To this end, the capacitor 31 of the branch III of the filter shown in FIGURE 1 is replaced by two seriescapacitors 34 and 35, whilst the branch IV which comprises the series-circuit of a capacitor 41, a resistor 42 and an inductor 43, is connected to the terminals of one of said capacitors, i.e. 35 in the figure. When such a filter is used in a television receiver the voltage derived from the terminals of capacitor 21 is applied to the input of the subsequent stage of the intermediate-frequency amplifier of the video-circuit, whereas the voltage set up at the terminals of capacitor 41 is applied to the input of the first stage of the intermediate-frequency amplifier of the sound-circuit. The amplitude of the signal applied to the last-mentioned amplifier increases as the suppression of the sound carrier wave in the filter increases.

In FIGURE 2 the neutralizing resistor is indicated by 40. The response curve A=(f) of the filter of FIGURE 2 is shown in FIGURE 5, from which it can be seen that this curve is in its response range substantially identical with that of FIGURE 4 but exhibits an angle ,6 which is approximately equal to 2.5 times the angle a of the lastmentioned figure. Due to this fact, a fairly satisfactory suppression of signals located in the comparatively narrow range of frequencies is also guaranteed with frequency shifts of the signals relative to the filter curve, provided such shifts are not too great.

FIGURE 3 shows one practical embodiment of a filter as shown in FIGURE 2, which is included between two intermediate-frequency stages of the video-circuit of a television receiver. This filter serves, on one hand, to eliminate the sound carrier wave from the video-circuit and, on the other hand, to apply said sound carrier wave to the sound circuit. The capacity 11 of the branch I is formed by the anode capacity of the tube 14, which precedes the: filter, and the; capacity 21 of the branch II is formed by the input capacity of the tube 24 whichfollows after the filter. The branch IV is coupled to the control grid of tube 44 of the first intermediate-frequency stage of the sound circuit.

Capacitors 50 and 50" serve as direct-current uncoupling capacitors.

FIGURE 3 shows the tubes 14, 24 and 44 as triodes. It will be evident that these tubes can be pentodes or can even be replaced by transistors.

It is to be noted that in FIGURE 3 the inductors 13 7 and 23, on the one hand, and 33 and 43, on the other,

are not inductively coupled to one another. As a result, the filter can beadjusted very easily.

In the filter shown the various branches are coupled together directly and by capacitive means 111 and IV). It will be evident that the branches can alternatively be coupled together by inductive means. a

What is claimed is:

1. A bandpass filter for passing a comparatively wide range of frequencies and also for strongly and selectively suppressing a comparatively narrow range of frequencieslocated within the said wide range or in the vicinity thereof, said-filter comprising first and second longitudinal branches connected in series and each comprising a seriesresonant circuit tuned to provide a low longitudinal impedance for said filter in said wide range, a transverse branch coupled to the common point of the longitudinal branches andcomprising a series-resonant circuit tuned toa frequency located in said comparatively narrow range of frequencies and input and output circuits connectedto points on said first and second branches other than said common point; wherein the improvement comprisesa fourth branch connected in parallel with only a portion of the transverse branch, saidfourth branch comprising a series-resonant circuit tuned to said frequency located inthe comparatively narrow range of frequencies whercby the range of suppression of said narrow range is increased.

2'. A bandpass filter for passing a wide continuous range of frequencies and selectivelysuppre'ssing a narrow range of frequencies adjacent said wide range, com-prising first and second series connected longitudinal branches and a transverse branch connected't'o the junction of said first and second branches; an inputcircuit connected to apoint of said first branch other than said function, an output connected to a point of said second branch other than: said junction, each of' said first and. second branches comprising a series resonant circuit tuned to provide a low longitudinal impedance of said filter in said wide range, said transverse branch comprising a series resonant circuit tuned toa frequency in said narrow range, where' by the impedance of said transverse branch is lbw at said frequency; and afourth branch connected in parallel with only a portion. of said transverse branch, said fourth branch comprising a series-- resonant circuit tuned tosaid frequency, wherebythe frequency range of suppressionin said narrowrange is increased.

31 The band'passfilterof claim 2; com-prisinga second output connected to'said fourth branch for receiving signals in said narrow range,

4. The bandpass filter of claim 2 in which said first and second. series branches are tunedto. substantially the same range of frequencies and also for strongly and'selectively suppressing a comparatively narrow'range offrequencies located" within the saidwide range or in. the vicinity thereof, said filter comprising first. and second longitudinal branches connected inseriesandeachcomprising a-seriesresonant circuit tuned to provide a low longitudinal impedance for said filter in said wider range, a transverse branch coupled to the common point of the longitudinal branches and comprising a series-resonant circuit tuned to a frequency located in said comparatively narrow range of frequencies, and input and output circuits connected to points on saidfirst and' second branches other than said common point, wherein the improvementcomprises. a fourth branch comprising a series-resonant circuit tuned to said frequency located in the comparatively narrow range of frequencies, the capacitor of. the transverse. branch being. comprised of the. series combinationof .twov

capacitors, and means connecting said fourth branch. in.

parallel with one of saidcapacitors.

7. A bandpass filterfor passing a wide continuous. range of frequencies and selectively suppressing a narrow range of frequencies adjacent said wide range, comprising first and second series connected longitudinal branches and a transverse branch. connectedtothe junction of said first and second branches, an input. circuit connected to a pointof: said first branch other than said junction, an output connected to. a. point of'said second branch other than said junction, each. of. said first and second branches comprising aseries resonant. circuit tuned to provide a lowlongitudinal impedance of. said filter. insaid wide range, said transversebranch comprising a series resonant circuit tuned to a frequency in said narrow range and being, comprised of the series circuit of aninductor; and. first; and second capacitor-s, connected: inthat order between said junction and: a point of reference potential; whereby the impedance; of said transverse branch is low at. said frequency; and a fourth branch connected in par" allelzwith: saidsecond capacitofisaid fourtli branch comprising a series resonant circuit tuned tosaidfrequency; whereby the frequency range of suppression said narrow range is increased.

References Gite'd UNITED STATES. PATENTS HERMAN KARLSAALBACH, Primary Examiner.-

LIEBERMKN; Examiner.

P. L. GENSLEKAssistant. Examiner. 

1. A BANDPASS FILTER FOR PASSING A COMPARATIVELY WIDE RANGE OF FREQUENCIES AND ALSO FOR STRONGLY AND SELECTIVELY SUPPRESSING A COMPARATIVELY NARROW RANGE OF FREQUENCIES LOCATED WITHIN THE SAID WIDE RANGE OR IN THE VICINITY THEREOF, SAID FILTER COMPRISING FIRST AND SECOND LONGITUDINAL BRANCHES CONNECTED IN SERIES AND EACH COMPRISING A SERIESRESONANT CIRCUIT TUNED TO PROVIDE A LOW LONGITUDINAL IMPEDANCE FOR SAID FILTER IN SAID WIDE RANGE, A TRANSVERSE BRANCH COUPLED TO THE COMMON POINT OF THE LONGITUDINAL BRANCHES AND COMPRISING A SERIES-RESONANT CIRCUIT TUNED TO A FREQUENCY LOCATED IN SAID COMPARATIVELY NARROW RANGE OF FREQUENCIES AND INPUT AND OUTPUT CIRCUITS CONNECTED TO POINTS ON SAID FIRST AND SECOND BRANCHES OTHER THAN SAID COMMON POINT; WHEREIN THE IMPROVEMENT COMPRISES A FOURTH BRANCH CONNECTED IN PARALLEL WITH ONLY A PORTION OF THE TRANSVERSE BRANCH, SAID FOURTH BRANCH COMPRISING A SERIES-RESONANT CIRCUIT TUNED TO SAID FREQUENCY WHEREIN THE COMPARATIVELY NARROW RANGE OF FREQUENCIES WHEREBY THE RANGE OF SUPPRESSION OF SAID NARROW RANGE IS INCREASED. 